I think you'll really like the cummins ISB. We also have two GSD's that come along with us. They really like being able to roam around while we're driving to our destination. The exhaust brake is important even on flat land.
I would not be afraid of a little delamination. How much are they asking for the unit? Should be in the $30-35k range. Get ready to sign the papers after the test drive. You won't believe how quiet and smooth it rides.
Oh, and regarding fuel mileage. The ISB in mine gets about 7.5 mpg in the mountains, hard on the throttle, climbing and towing a CRV. On flat ground, 60 mph, I have gotten around 10 mpg. 60 mph is the sweet spot for mileage.
